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The Yoruba are a large ethnic group who live in south-western Nigeria, parts of Benin, Togo and Ghana.

A 2011 study found that “the high twinning rates that were known to exist in this country [Nigeria] turn out to be the dominant pattern in the whole Central African region”.

Researchers from Radboud University in the Netherlands and Oxford University’s sociology department found that Africa has the world’s highest twin-birth rates. “A zone with high twinning rates… runs from Guinea in the West along the Atlantic coast to [the Democratic Republic of the Congo] and then crosses the continent to Tanzania, Mozambique and the Comoros.”

A comparison of data from Demographic and Health Surveys of 75 developing countries collected between 1987 and 2010 revealed that Benin had the highest national twinning rate of 27 sets of twins per 1,000 births while Nigeria had a rate of 19 sets of twins per 1,000 births.

This study, however, looked at births recorded in countries and not among ethnic populations.

Earlier research by Italian academics shows that spontaneous multiple births are more common in black populations than white ones and very low among Asian populations.

They speculated that it’s due to possible genetic predisposition “inherited primarily through the mother”.

They Radboud/Oxford study concluded that maternal age is the “most important factor associated with twinning… The number of twin pregnancies increases substantially with maternal age.”

Do Yoruba people have the world’s highest twin birth rate? As we were unable to find ethnicity-specific twin birth data, we rate this claim unproven. (Note: We will update this report if we locate new or better information.)

Claim: “Eating yam increases the chances of having twins.”Verdict: unproven

A book about twins in African cultures shows that some African societies view the birth of twins as a burden to their families while others see it as a blessing.

Lagos-based gynaecologist and obstetrician, Prof Oladapo Ashiru, told Africa Check that the yam claim is common and that twin birth rates are particularly high in the Nigerian town called Igbo-Ora in Oyo state.

The town which is about 142 km north of Lagos, has previously been dubbed “the land of twins”.

It has been widely reported that community members believe that the prevalence of twins in their society is largely due to the food they eat. More specifically, they believe that their high consumption of yam is the reason for the significant rate of twinning in Igbo-Ora.

Yams are tuber vegetables like potatoes, Ashiru explained. The crop is grown in many parts of west and central Africa.

A 2008 study noted that yams are believed “to contain a natural hormone phytoestrogen, which may stimulate multiple ovulation”.

But the idea that eating yam increased the chances of having twins is “speculation”, Ashiru warns.

Vice-president of the Southern African Society of Reproductive Medicine and Gynaecological Endoscopy, Dr Sulaiman Heylen, said it is unknown whether yams can increase the chances of a twin birth.

“There is no scientific evidence that yams, or any other product or food, can increase a woman’s chances of having twins,” Heylen told Africa Check.